Due to the constant increasing demand for potable water for drinking and irrigational use, seawater desalination maintains its importance. Furthermore, economically feasible and large scale seawater desalination is specifically important because of the continuous growth in the population and related growth of the industry. Even though, membrane based desalination is less energy intensive process, compared to thermal desalination, the energy consumption is still high and needs to be lowered with a more environmentally friendly and economically feasible desalination process.
Several methods for less energy intensive desalination have been developed including forward osmosis and harvesting of hydraulic energy from a direct osmosis process (PCT/US02/02740, PCT/ES2011/070218). Despite its theoretical energy free mechanism, no feasible applications have been found for forward osmosis. The main problem with the forward osmosis is the extraction of the drawn water from draw solution and the recovery of the draw solution with a continuous and feasible process. On the other hand pressure retarded osmosis (PRO) has been proven to be more than promising process for energy production and recovery process (Statkraft Osmotic Power Pilot Plant, Norway). However, an innovative engineering design for the maximum energy recovery and/or production is required for the commercialization of the PRO process.
The driving force of the osmotic process is the osmotic pressure difference between the two aqueous solutions on the opposite sides of the semi-permeable membrane. Osmotic pressure of an aqueous solution can be calculated by using Van't Hoff relation:π=θ·v·c·R·T. 
where, v is the number of ions produced during dissociation of the solute, θ is the osmotic coefficient, c is the concentration of all solutes (moles/l), R is the universal gas constant (0.083145 l·bar/moles·K), and T is the absolute temperature (K).
The water flux through a semi-permeable membrane by osmotic pressure difference is given as (McCutcheon and Elimelech, 2007):Jw=A(πD,b−πF,b)
where, Jw is the water flux through the semi-permeable membrane, A is the pure water permeability coefficient of the semi-permeable membrane, πD,b and πF,b are the bulk osmotic pressures of draw and feed solutions, respectively.
PRO can be used to generate or recover energy (power) by utilizing the Gibbs free energy of mixing with respect to the salinity difference of two aqueous solutions (Sandler, S. I., 1999, Chemical Engineering Thermodynamics, 3rd ed.; Wiley).−ΔGmix=RT{[Σxi ln(γixi)]M−θA[Σxi ln(γixi)]A−θB[Σxi ln(γixi)]B}
where, xi is the mole fraction of species i in solution, R is the gas constant, T is temperature, and γ is the activity coefficient of the species.
In a PRO system, a constant hydraulic pressure is applied on the high salinity aqueous solution and permeation of water from low salinity aqueous solution continues while the osmotic pressure difference of two solutions is higher than the applied hydraulic pressure. Pressure of the high salinity aqueous solution can be conserved with the additional energy from Gibbs free energy of mixing while the volumetric flux of the solution increases. Yip and Elimelech (2012) found that the highest extractable work in a constant-pressure PRO process is 0.75 kWh/m3 when seawater and river water were used for draw and feed solutions, respectively. Therefore harvested Gibbs free energy of mixing, in terms of pressure and volume, can be used to produce energy and/or recover pressure.
In case of energy production; a water turbine can be used to generate power by utilizing the pressure and volumetric flux of the aqueous solution. Even though the modern Pelton turbines can reach up to 92% efficiency, the average efficiency is generally around 90%.
In case of pressure recovery; there has not been an engineering application to use the harvested Gibbs free energy of mixing with a PRO process for pressure recovery of a membrane desalination process. Modern seawater reverse osmosis processes use pressure exchangers to recover pressure from the brine and to pre-pressurize the seawater before entering the RO process. In this way, up to 60% of the required energy for pressurizing the seawater for RO process can be saved. Modern pressure exchangers, such as isobaric pressure exchangers, can reach an efficiency of 97%. Therefore pressure recovery can be a better alternative than energy production for membrane based seawater desalination because of its higher recovery efficiency.